1. Field of the Invention
The invention relates to the use of remote controllable circuit breakers with positive temperature coefficient resistivity (PTC) elements and reduced size and weight thermoplastic cases.
2. Background of the Art
Remote controllable circuit breakers are widely used for the interruption of electrical current in power lines upon conditions of severe overcurrent caused by short circuits or by ground faults. The remote signal is, for example, transmitted from a personal computer hundreds of miles away. The prior art circuit breakers include disadvantages such as, a very large size and high costs.
FIG. 1 (prior art) is a longitudinal sectional view of a typically remote controllable circuit breaker 10 for interrupting the flow of electrical current in a line. The circuit breaker 10 is, for example, the QOAS.TM. circuit breaker, manufactured by Square D Company, which is large in size and weight and very costly to manufacture. The circuit breaker 10 can be turned on or off by a remote signal while the breaker in the "ON" position. The remote control function of the circuit breaker 10 is of FIG. 1 is accomplished using a small motor 12, which is a very expensive part in such circuit breakers. When the circuit breaker 10 in the "ON" position, current is received at the line terminal 14 and passes through the two closed contacts 16 and 18, respectively. The contact 16 is welded onto blade 20. The current passes through the blade 20 to a bimetal 22, and leaves the circuit breaker 10 through terminal 24 and lug 26. The circuit breaker 10 includes an operating handle 11 and a spring 21 connected to the blade 20.
When a remote signal to turn the circuit breaker 10 off is received by printed circuit board 28, the motor 12 rotates driver 30 counter-clockwise. A sector gear 32 translates the rotation into a displacement of a lever 34. The lever 34 pulls the blade 20 and separates the contacts 16 and 18, respectively. The current is then interrupted or "turned off", and the circuit breaker 10 remains in the "OFF" position until another remote control signal is received to turn the circuit breaker 10 on. When a remote control signal to turn the circuit breaker 10 on is received by the printed circuit board 28, the motor 12 rotates clockwise. The sector gear 32 forces the lever 34 to push the blade 20 and close the contacts 16 and 18, respectively, wherein the flow of current may be resumed.
The circuit breaker 10 includes conventional technology, such as the bimetal 22 used for overload protection. When the circuit breaker 10 is in an overload situation, such as 135% of the rated current, the high current brings additional heat to the bimetal 22. The bimetal 22 is deflected by the heat and causes a trip lever 36 to detach. Circuit breakers using bimetal for overload protection must be calibrated. The calibration is performed using screw 38. Calibration of the bimetal circuit breakers typically causes problems, and the manufacture of these types of circuit breakers including the bimetal is costly. The bimetal used in the circuit breakers does not behave consistently, even after calibration, and therefore, some circuit breakers will not trip at the rated 135% overload situation. In addition to many of the other problems associated with the use of bimetal in circuit breakers is terminal cracking, particularly in miniature circuit breakers. The calibration of the miniature circuit breakers also results in high stress of the load terminal.
For short circuit interruptions, the prior art circuit breaker 10 uses arc stacks 40 and a large arc chamber, large contacts 16 and 18, and a large separation between the two contacts after the circuit breaker 10 trips. One of the problems associated with the process of interruption of the current during severe overcurrent conditions is arcing. Arcing occurs between the contacts of circuit breakers used to interrupt the current, which is highly undesirable for several reasons. Arcing causes deterioration of the contacts or blades of the breaker and causes gas pressure to build up. Arcing also necessitates circuit breakers with larger separation between the contacts in the open position to ensure that the arc does not persist with the contacts in the fully open position. In the circuit breaker 10 of FIG. 1, the large components and designs are used because almost 100% of the interruption energy becomes arcing, which generates high interruption pressure during a short circuit interruption. At least six rivets 42 are typically used in the prior art circuit breaker 10 design to hold the circuit breaker cover and base together because of interruption pressure. The interruption pressure also causes damage to end use equipment.
Another disadvantage in the prior art circuit breaker 10 design involves the mag-trip function. If the current through the circuit breaker 10 reaches a value higher than a predetermined value such as, for example, approximately 500% of the ampere rating, the circuit breaker 10 trips before the bimetal 22 has a chance to deflect. The predetermined current value is the mag-level of the circuit breaker 10. An armature 44 and yoke 46 provide the tripping function. Under normal conditions, there is an air gap between the armature 44 and the yoke 46. When the current reaches the predetermined mag-level, the armature 44 is pulled to the yoke 46 to close the air gap. The trip lever 36 is then delatched and the flow of electrical current in the line is cut off instantaneously by the circuit breaker 10. However, the prior art designs of the armature 44 and yoke 46 cannot ensure consistent mag-levels among a batch of the same circuit breakers. The standard deviation of the mag-level of the prior art circuit breakers is too large to consistently protect circuits.
The prior art circuit breakers include disadvantages such as, a very large size and high costs. In order to hold the existing circuit breaker 10 mechanisms, such as the motor 12 and tripping mechanisms, the circuit breaker 10 base and enclosure (not shown) is designed with a very large size. The motor 12, the large contacts 16 and 18, the arc stacks 40 and the calibration of the bimetal 22 all contributes to the costly manufacturing of the existing circuit breaker 10 design. The thermosetting material used in manufacturing the base (not shown) and cover 48 of the circuit breaker 10 is also costly, especially compared to the manufacturing and use of thermoplastic cases. Other disadvantages in the prior art circuit breaker design include mechanical variations, and wear and contamination of parts.
Chen (U.S. Pat. No. 5,629,658) discloses a number of devices in which PTC elements are used in conjunction with two or more switches to limit the current under short circuit conditions and thereby reduce the associated arcing. U.S. patent application Ser. No. 08/918,768, filed Aug. 25, 1997 (Chen et al.) also discloses a number of devices in which PTC elements are used in conjunction with two or more switches to limit the current under short circuit conditions.
There is a need, therefore, for a circuit breaker design which is less costly to manufacture, is more reliable across a batch of circuit breakers manufactured and is of a much smaller size overall.